The present invention relates to a measuring device, which uses a strain gauge. More specifically, it relates to a device making it possible to perform more accurate measurements than the best prior art devices, whilst being much less costly than the latter. This invention has numerous applications in the field of metrology and particularly electronic weighing.
The best known existing method for obtaining strain gauges consists of using a thin polyimide film to which is stuck a very thin sheet (approx. 5 .mu.m) of a resistive material based e.g. on nickel, chromium or copper. The resistive coating is then made into fine strips in order to obtain a resistance of suitable value and form, the gauge then being directly connected by adhesion to the test body.
Although such gauges make it possible to carry out very accurate measurements (up to 10.sup.-4 of the effective range), their manufacture is complicated and difficult. Thus, the handling and adhesion of very thin sheets is far from easy and it is even more difficult to manufacture sheets with a thickness of even 5 .mu.m. Thus, such gauges are very expensive and it is difficult to envisage their use in mass-produced sensors.
Another method consists of producing the gauge directly on the test body by the vacuum deposition of an insulating layer, followed by a resistive layer. The use of a mineral insulant with a very limited thickness gives the sensor a very good stability and good metrological properties, but this method also suffers from disadvantages. The surface of the test body on which the deposit is made must undergo very careful polishing to prevent any holes in the insulant. Furthermore, with the output of the machines used expressed in surface units per time unit, the cost of the sensors is proportional to the surface of the test body on which they are placed and the latter is sometimes relatively large for mechanical strength reasons. Finally, due to the sensitivity of the test body, to both chemical products and heat treatments, the production process can be very complicated, which considerably increases the cost thereof.
Finally, a third method uses as the test body a monocrystalline silicon sheet in which resistors are produced by the diffusion of a doping agent as in integrated circuits. Although this method permits mass production, the sensitivity of silicon to temperature changes does not at present make it possible to obtain precisions better than 10.sup.-2 of the effective range.